A digital video image comprises an array of pixels, with a known number of pixels in each horizontal row and vertical column according to an associated image format. Oftentimes the image is to be displayed in a system that uses a different number of pixels in each row and/or a different number of columns from that of an input image. Accordingly, it is often desirable to scale the image for display in the new system. The particular scaling in the horizontal and vertical dimensions can be different based on the dimensions of the input image relative to the desired dimensions of the output image. Scaling also is utilized for zooming of images and/or for displaying such images in windows having freely definable size and position. Various approaches have been developed for scaling, which can vary according the source of the input video signal and the desired output format.
In general, a video signal is scaled spatially in two dimensions, namely horizontally and/or vertically. The scaling can be implemented by simple window-cropping, direct decimation, or direct repetition. One example of an existing scaling technique is to apply weighted averages to neighboring image pixels to scale a PC graphics image both vertically and horizontally to match a television resolution format. To help reduce flicker, a scan converter having frame storage capabilities sometimes is used. While this frame-based flicker reduction can improve image quality, such associated with television images, as well as have extra flexibility in its graphics interface requirements, it is not cost-efficient due to the large size required on a silicon device to provide adequate frame storage. Another type of flicker reduction technique attempts to limit each display point (pixel) on a line of an interlaced field to be vertically paired with the point (pixel) on the vertically neighboring line of the next field. Still another approach utilizes low-pass filters to filter out vertical high frequency components of original graphics in order to generate relatively smooth vertical contrast between neighboring pixels. This low-pass filtering process may include line averaging or a similar technique.
Different types of video signals have different frame rates for display on computer screens, television screens and film. For example, film material typically utilizes frame rates of 24, 25 and 30 Hz, and video usually employs frame/field rates of 50 and 60 Hz. Television displays are commercially available with picture rates of 50, 60 and 100 Hz, and can employ progressive or interlaced scanning. Simple picture rate converters repeat pictures until the next arrives, which can result in blur and/or judder when motion occurs. Similarly, de-interlacing sometimes results from repetition, or averaging of neighboring lines. The more advanced deinterlacing concepts further apply vertical-temporal processing, but even these can degrade those parts of images where motion occurs. Further complexities arise for conversion from a non-interlaced to an interlaced format, which if not accounted for can reduce imagerial details during conversion and/or to cause introduction of artifacts.